The present invention relates to a voltage detector utilizing the electrooptical effect.
FIG. 1 shows a conventional voltage detector 1 which uses an electrooptical amplitude modulator 4. Input light from a light source 2 is modulated by the electrooptical amplitude modulator 4 in accordance with an electrical signal (i.e., voltage) being provided thereto. Output light from the modulator 4 is detected by a photodetector 6. An output signal of the photodetector 6 is subjected to necessary processing in a processing device 7, and then displayed on a display device 8.
The electrooptical amplitude modulator 4 consists of an optical modulator 4B which receives the electrical signal to be measured, and a polarizer 4A and analyzer 4C disposed on the input and output side of the optical modulator 4B, respectively. As shown in FIG. 2, the electrooptical amplitude modulator 4 utilizes the following relationship between the input voltage V and the output light intensity I: EQU I=I.sub.o sin.sup.2 {(.pi./2)(V/V.sub..pi.)} (1)
where I.sub.0 is the input light intensity and V.sub..pi. is the half-wave voltage of the electrooptical amplitude modulator 4. Further, it is assumed that attenuation due to light absorption does not occur in the modulator 4.
If a high-speed photodetector, such as a streak camera, is employed as the photodetector 6, a high-speed voltage detector can be constructed.
In the above-described voltage detector 1, since the output light intensity I is not proportional to the voltage V being applied, the output signal of the photodetector 6 should be subjected to a conversion process based on equation (1) in the processing device 7 to obtain a signal properly representing the input voltage.
Where the input voltage is small, the resulting output light intensity from the electrooptical amplitude modulator 4 is also small, preventing accurate voltage measurements. Further, there is a problem that noise in the light intensity of the light source 2 deteriorates the S/N ratio of the output signal.